Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Abstract

Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x)(0.63B 2O 3 · 0.37Li 2O) · xFe 2O 3, with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe 3+ concentrations (Fe 2O 3 molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g ef = 4.3 peak accompanied by a shoulder continuing down to g ef = 9.7, are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe 2O 3 contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe 3+ ions. It is concluded that the spectrum is due to diluted Fe 3+ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe 3+ ions is estimated as 4.9 Å. A diluted state of Fe 3+ ions in the glass network in this range is also confirmed by the temperature dependence of the g ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe 3+ ions (Fe 2O 3 molar contents from 0.01 to 0.1), the width of the g ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g ef ≈ 2 line, which coexists at these concentrations with the g ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe 3+ ions. The temperature dependence of the g ef ≈ 2 line intensity in 0.1 mol Fe 2O 3 glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe 2O 3 glass, which is attributed to an appearance, at higher Fe 2O 3 contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g ef ≈ 2 line in the low-temperature EPR spectra.